Assault weapons and hand guns are seen for sale earlier this month. The National Rifle Association and other gun rights groups say that good guys with guns are the nation's best protection against armed criminals.

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Washington — How often do Americans use firearms in self-defense? That’s a basic question whose answer could shed much light on the gun control discussion now swirling through Washington.

The National Rifle Association and other gun rights groups say that good guys with guns are the nation’s best protection against armed criminals. Does the data bear that out?

Well, the short answer is there is not much data to go on. So we don’t really know for sure. For years, public policy researchers have pointed to self-defense as an aspect of gun use that needs much more study.

“While a large body of research has considered the effects of firearms on injury, crime, and suicide, far less attention has been devoted to understanding their defensive and deterrent effects,” concluded an in-depth 2005 National Research Council study of the state of firearms and violence data.

The data that does exist varies widely. According to Bureau of Justice Statistics numbers, each year between 1987 and 1992 about 62,200 victims of violent crimes used guns to defend themselves, while another 20,000 annually used guns to protect property. According to the National Self-Defense Survey conducted by criminology professor Gary Kleck of Florida State University in 1993, Americans used guns 2.3 million times a year to defend themselves between 1988 and 1993.

That’s a pretty big spread. As a 2012 Congressional Research Service report on gun issues points out, law enforcement agencies do not collect self-defense information as a matter of course, and the available research thus depends on limited numbers of surveys and other self-reported information.

“Self-defense” is also a difficult item to define. It’s a term that covers not just obvious cases, such as the use of a firearm to repel an armed home invasion. To some survey respondents, openly carrying a weapon through a dangerous neighborhood might count as successful self-defense.

In addition, what some might categorize as defense, others would judge aggression. This can easily be seen in the Trayvon Martin case, where armed neighborhood watch member George Zimmerman shot an unarmed teenager because he felt his life was endangered. Mr. Zimmerman’s trial on second-degree murder charges is set for this June.

According to a 2002 study published in the journal Justice Quarterly, 27 percent of 297 defensive gun uses surveyed may have been unnecessary or “even exceeded a defensive purpose.”

Indeed, a separate 2004 survey of reported gun self-defense use by California teens found that “most of the reported self-defense gun uses were hostile interactions between armed adolescents.”

Again, these findings should be seen as preliminary, given the shaky nature of the foundational use data and their reliance on self-reporting and surveys. They’re as much indications pointing the way to more research as conclusions in and of themselves.

Some other surveys conclude that in general the use of guns in self-defense does reduce the likelihood of property loss or injury. A 2004 study in the journal Criminology, based on the previously mentioned Bureau of Justice Statistics numbers, concluded that about 10 percent of those individuals who used guns in self-defense were subsequently hurt.

“Compared to lack of resistance, self-protection reduced the likelihood of property loss and injury,” concluded the study.